This invention relates to a system and method for safely parking a read/write head of a disk drive in the event of a power down condition and more particularly, to such a method that enables the parking of the read/write head on a load/unload ramp, irrespective of the position of the read/write head upon the occurrence of the power-down condition.
In the past, most disk drives have parked their read/write heads on the magnetic disks when the drives were powered off. This type of process is known as contact start-stop (CSS). In a CSS disk drive, just before the disk(s) stop spinning, and when they initially begin to spin, the heads are in contact with the disk. This generates wear at the interface between heads and disks, and limits the lifetime of the disk drive.
Note, that while the term xe2x80x9cheadxe2x80x9d is used hereafter, the term xe2x80x9csliderxe2x80x9d can be substituted. A slider is a block of hard (usually ceramic) material that carries the actual read and write head elements. A slider allows the head to fly in close proximity to the disk through the action of an air bearing.
Alternatively, the heads can be removed from the disk during power-down by using a load/unload ramp at the outer diameter of the disks, as shown in FIG. 1. In FIG. 1, a disk drive 10 is shown, which contains one or more magnetic disks 12 that spin about a spindle 14. Data is written with onto disks 12 by a read/write head 16, and information is read back using the same read/write head 16. Read/write head 16 is attached to the bottom of a suspension 18 which is part of an actuator 20 that rotates about a pivot point 22. Actuator 20 is moved by current through a voice-coil motor 24. Crash stops (26 and 28) limit the travel of actuator 20. FIG. 1 shows a load/unload ramp 30 onto which a tab 32 at the tip of suspension 18 is pushed before disk(s) 12 stop spinning in order to remove read/write head(s) 16 from disk(s) 12. A flexible cable 29 connects actuator 20 to a connector 31.
A close-up view of load/unload ramp 30 is shown in FIGS. 2 and 3. While only a single disk and two heads are shown, multiple disks and multiple sets of heads are also possible. Disk 12 is mounted on spindle 22 and is rotated by a spindle motor 40. Read/write heads 16 are attached to suspensions 18 that contain tabs 32. Before disk 12 is stopped, tabs 32 are pushed onto load/unload ramp 30 which is attached to base plate 42 of disk drive 10 by a mounting screw 44 or other fastening means. He ads 16 are lifted off disk 12 by tab 32 on the end of suspension 18 that travels up load/unload ramp 30.
FIG. 3 is a top view of the arrangement shown from the side in FIG. 2 and illustrates the direction 46 which tab 32 moves when the tip of suspension 18 is pushed onto load/unload ramp 30 before disk 12 is stopped.
In the case of a system-initiated shutdown, with power still applied to the drive, the position and state (velocity) of the actuator can be found by reading servo information present on the disk. The actuator can then be stopped by the disk drive electronics, after which it can be moved onto the ramp in a controlled manner.
A more difficult case, and the one addressed by this invention, is one where the power to the disk drive is interrupted. This can happen as a result of a power failure, unplanned system outage, or physical removal of the drive from the system.
In prior art CSS disk drives, the actuator is usually pushed towards the inner diameter (ID) of the disk, into an ID crash stop (a compliant element that usually contacts the actuator at the end away from the heads to limit its travel). This action is accomplished by passing a current through the voice coil motor that controls the actuator, in the appropriate direction. This current is often generated by rectifying a back electromotive force (back-emf) from the spindle motor that spins the disks. Since the disks will still be spinning immediately after the power is cut due to their rotational inertia, the spindle motor acts as a generator and creates a back-emf across the spindle motor terminals. This back-emf is periodic in nature, and is therefore rectified to provide a control voltage.
The same method cannot safely be used in a load/unload disk drive, because high-speed impacts of the actuator with the ramp can cause permanent head, disk, ramp, or suspension damage. If the disk drive is in the middle of a seek when the power is cut, the heads can be traveling as fast as several meters per second, towards or away from the load/unload ramp.
In U.S. Pat. No. 4,237,501 to Barmache et al., a system is described for enabling an emergency head unload action in the event of a power down. A control circuit performs a dynamic braking action by electrically shorting a positioning coil to reduce the radial velocity of the head to zero. Then a current is applied which moves the head towards the disk""s outer edge, followed by a further current to enable the head to climb an unload ramp. While the electrical shorting action of Barmache et al. will eventually achieve the velocity reduction, it is relatively slow acting. Further, there is no way of knowing where the head is actually positioned when its radial velocity drops to zero. Accordingly, it is difficult to assure that a correct current value is thereafter applied which will safely move the head to the unload ramp, without damage.
U.S. Pat. No. 4,679,102 to Wevers et al. describes a system for enabling an emergency head unload action in the event of a power down. The back-emf is used to control a stepper motor to move the head to a park position at the inner diameter of a disk. Since the position of the head may not be known, the timing of the pulses to the stepper motor is such that a sufficient number of pulses are applied to move the head to the parking zone, even if the head is present over the outer-most disk track. In a disk drive with a stepper motor instead of a voice coil motor, the actuator automatically stops when power to the drive is cut. The clocking method of Wevers et al. therefore cannot be applied to a disk drive with a voice coil motor.
Accordingly, it is an object of this invention to provide a method and apparatus for moving a transducer head to an unload position in the event of a power down action.
It is a further object of the invention to provide a method and apparatus for moving a transducer head to an unload position in the event of a power down action, wherein the head is moved in a controlled manner that avoids damage to the head-arm structure.
It is yet another object of the invention to provide fast acting method for moving a transducer head to an unload position in the event of a power down action.
A disk drive includes a read/write head mounted on an actuator, a disk having an inner diameter region and an outer diameter region, the head being radially movable by the actuator between the inner diameter region and the outer diameter region. An unload ramp is positioned at the outer diameter region. A controller outputs control signals upon sensing a power down condition. A feedback circuit is adapted to sense a back emf generated by the actuator and to use the back emf to apply a counter-acting voltage to the actuator that tends to bring the actuator to a stop. A switch is responsive to the control signals to (i) connect the feedback circuit to the actuator to enable the actuator to be brought to a stop while the disk continues to rotate and (ii) for then causing the actuator to move the head to the unload ramp. The switch further applies a voltage which causes the actuator to bring the head to the inner diameter region of the disk, before causing the actuator to move the head to said unload ramp.